This invention relates to feedback networks for linear amplifiers. One of the major disadvantages of linear solid state amplifiers is that intermodulation distortion performance is poor in comparison to their vacuum tube counterparts. It is generally difficult to apply RF feedback around multiple stage of broadband solid state linear amplifiers. The phase shift associated with these amplifiers varies too widely to consider any type of phase compensation network in the feedback loop. Without feedback, of course, the amplifier performance is limited to the capabilities of the transistors or tubes in the amplifier gain chain. The performance of an amplifier, in the Radio Frequency (RF) range, is measured by the intermodulation distortion produced by the application of two equal amplitude signals or tones, generally separated in frequency by 1 KHz, to the amplifier. Acceptable performance of the amplifier is when the intermodulation distortion is greater than 25 to 35 db below either tone and gain variation on the order of 6 db. It is generally accepted that application of feedback will improve the performance and reduce gain variation depending upon the amount of feedback employed. Traditionally, 10 db of feedback should improve the intermodulation distortion level by 10 db.
The problem was addressed in U.S. Pat. No. 3,777,275 in which non linear amplifying devices were used to produce bandpass linear amplification of a signal having amplitude variation. The input signal was transformed into two constant amplitude phase modulation components which together contain in their phase fluxuation the total information content of the input signal. The components were amplified separately by devices which preserved phase, and the recombination of the amplified components reproduced a linear amplified replica of the original signal input. This technique was primarily useful at high frequency, but also could be modified to provide a frequency translation.
Similarly in U.S. Pat. No. 3,909,742, hybrid combiners, nonlinear amplifying devices, phase varying devices and an attenuator were used to produce a linear amplified replica of a bandpass analog signal having amplitude variations. The original analog input signal and a feedback signal are applied to a hybrid combiner which subtracts the feedback signal from the original input signal to produce one of two components of equal amplitude and adds a feedback signal to the original input signal to produce the other component. The components are amplified separately by nonlinear amplifying devices and the amplified components then are recombined by a second hybrid combiner which produces two outputs. One output, the difference between the two components is the linearly amplified replica of the original signal. The other output, the sum of the two components, is the feedback signal which is applied through a series combination of the phase varying devices and the attenuator to the first hybrid component.
The problem of distortion in the use of feedback circuits to reduce or eliminate distortion was discussed for tube type amplifiers in Chapters 13 and 14 of the book entitled, "Signal Sideband Principles And Circuits" by E. W. Pappenfus, W. B. Bruene and E. O. Schoenike, published by McGraw-Hill.